The present invention relates to liquid-based systems containing particles and, more specifically, to systems and methods for characterization of particle components in mixtures.
Many processes and products include liquid media containing undissolved particles. The quantity and character of such particles in the media may be by design, may be an uncontrollable attribute, or may be a by-product resulting from the use of the media in a process such as cooling, lubricating, polishing or cleaning. The efficacy of a process or product incorporating such particles may be optimum when the concentration or character of such particles meets predetermined, albeit sometimes ideal, criteria.
The types of media which contain undissolved particles span a wide range of both industrial and consumer applications. Examples include blood, consumable liquids, industrial wastewater, lubricants and slurries, to name just a few. A specific example, for which exemplary embodiments of the invention are described below, is an abrasive slurry of the kind used for polishing a surface. It is desirable to characterize particle size and concentration in such media in order to provide more predictable abrasive properties. However, in some applications, the distribution of particle size and concentration is quite variable. Changes in these parameters are often very difficult or cumbersome to monitor, particularly during manufacturing processes. In some instances particle size distribution is so unstable as to cause unacceptable inconsistency in the process results. The instability is at times an inherent feature of the mixture. Controlling, or at least monitoring, the dynamics will also render processes more predictable and repeatable, thus adding further economic efficiency to the manufacture of products.
In abrasive media, it is not uncommon for the distribution of particle sizes to deviate from a preferred value. Under these circumstances, it is desirable to perform one or more inspections to assure such deviation is within acceptable tolerances. Initial inspections may use conventional analytical tools such as microscopy. However, the constraints of in-use monitoring may preclude application of the same analytical tools and thus such inspections may be limited to a form of visual observation. Observation of accumulated residue material is an indirect indicator that the composition of a mixture has changed.
Mixture instability is especially problematic in volume manufacturing operations, i.e., those requiring a high level of repeatability from one work piece to another. For example, when a timed polishing operation is expected to provide a predictable result, a change in the frictional characteristic of the polishing medium may so alter the polishing rate as to result in excess removal of material from the work piece or, perhaps, a nonuniformity across the workpiece. In many manufacturing applications such variations in a media characteristic occur at a pace which requires in-line monitoring. However, as already noted, conventional monitoring techniques are not well-suited for in-line applications.
Moreover, as the specifications for process steps become more demanding, conventional techniques may not have the ability to adequately resolve unacceptable variations. On the other hand, if attributes of liquid media, such as particle size distribution, rate of sedimentation and rate of agglomeration, were known with sufficient accuracy, then a wide variety of processes which utilize particles suspended in liquid media could be deployed with increased precision. There exists a need to improve the speed at which quantitative characterizations can be performed in order to provide relatively accurate and time-responsive results.
Further, it is often desirable to minimize, or at least monitor the size and quantity of, particles which enter lubricating media in order to sustain desirable properties. Thus, when the concentration or size of particulate matter rises above certain levels, the lubricant can be replaced or reconditioned. More generally, it is desirable to monitor entry of nonsoluble particles in liquid media. Suitable in-line techniques for continuously monitoring these characteristics would assure timely replacement of materials, facilitate maintenance activities and extend the life or efficiency of equipment.
To address the above-discussed needs the present invention provides a method for monitoring a characteristic of a mixture comprising a plurality of particles suspended in a liquid medium. Optical systems are provided for monitoring a colloidal suspension. An interferometric system and a method of characterizing particles in a colloidal suspension are also provided.
According to one embodiment of the invention, a method is disclosed for monitoring a characteristic of a mixture comprising a plurality of particles suspended in a liquid medium. First and second portions of radiation are generated from a source having a coherence less than one. Interference signals are generated by combining the portions after the first portion traverses a reference path and the second portion traverses a path extending into the mixture.
According to another embodiment there is an optical system for monitoring a colloidal suspension. The system includes a chamber positioned to receive a sample of the suspension while the suspension is circulating in a mechanical system. An interferometer is positioned to generate interference patterns developed by a combination of a reference radiation signal with a sample radiation signal indicative of a characteristic in the suspension.
An interferometric system includes first, second and third optical fibers with first and second couplers. The first coupler is configured to receive radiation through the first fiber, provide a first radiation portion through the second fiber and provide a second radiation portion through the third fiber to the second coupler. The system also includes fourth and fifth optical fibers each configured to receive radiation from the second coupler and transmit radiation for receipt by the other.
In a method for characterizing particles in a colloidal suspension, during a period in which the particles undergo sedimentation, changes are measured in the intensity among interference patterns generated by combining first and second partially coherent radiation portions. Each radiation portion is generated from the same source. The first portion includes a component transmitted through a fraction of the suspension, and the second portion is transmitted through a reference path.